1. Field of the Invention
This invention relates to a linear-type burner assembly capable of discharging uniform heat over a long span, and is more particularly concerned with a burner assembly that has high combustion efficiency and requires minimum excess air for combustion. The burner assembly's design will not allow flame retrogression through the burner apertures, even under extreme operating conditions, which normally would have resulted in flashback in the mixture manifold. The burner can be operated at any angle around its longitudinal axis, while maintaining stable combustion.
2. Description of the Prior Art
There are many types of gas burners used in industrial heat processing, including packaged burners, air stream burners (make-up air type) and line burners. With the exception of line burners, most industrial type burners are rated at 500,000 BTUH or higher. For the present intended use of these types of burners, it is usually advantageous to keep the maximum input as high as practical and still achieve complete combustion. In most oven applications, a recirculating air system is used to distribute the heat energy from these high BTUH burners to the oven environment. In other words, in most burners of today's technology there is a concentrated discharge of energy and a means must be provided independently from the combustion air supply to the burner to uniformly distribute the energy.
An example of a present line burner is the LINOFLAME.TM. gas burner manufactured by the Maxon Corporation. These burners utilize a gas/air manifold that is an integral part of the burner structure. The sections of these burners are intended to be assembled together, and the total amount of the gas/air mixture required downstream of any burner must pass through the manifold of that burner. Therefore, it is not practical to assemble these types of burners in lengths longer than 7 ft. to 10 ft. because of the high mixture velocity which would affect the distribution of the mixture passing through the first several burners of a series of burners. Beyond a length of 7 ft. to 10 ft., the burners need to be broken into separately fed, shorter lengths (connected by cross-ignition end plate sets) to minimize burner distortion and stresses during alternate heating and cooling cycles. Also, the line type burners of present day technology have to be carefully matched to the equipment supplying air/gas premixture.
There are other disadvantages associated with present-day line burners. First, most line burners employ a premixture of the gas and air, and therefore, if for some reason the flame retrogresses into the mixture manifold, a fire or an explosion, referred to as flashback, could occur. The work by me leading to the development of the burner assembly of this invention has included the investigation of laminar and turbulent flame flashback in mixtures of methane and air and propane and air in high temperature environments (400.degree. F. to 1700.degree. F.). Many factors influence flashback in a nozzle or burner port. It has been shown that flashback can be controlled to a large extent simply by a cooling process. A method utilized by present line burners to reduce flashback is to use raised burner ports. If the surfaces of the raised ports are kept cool during the combustion process, the flame will not penetrate into the ports beyond a distance of a few millimeters corresponding to the heated zone of the port rim. Also, the ratio of the interior diameter to the exterior diameter of the raised ports influences flashback. The dead space (the space between the flame base and the burner surface), the mixture temperature, and the fuel and air mixture ratio also affect flashback in methane/air or propane/air (gas/air) mixtures.
While all of the above factors influence flashback, it is widely accepted, and has been demonstrated in studies I have conducted, that the critical boundary velocity gradient of the gas/air mixture is a primary controlling factor in flashback. When the gas/air mixture velocity exceeds the flame velocity as it exits the burner port or aperture, the flame has a lifting tendency. When the gas/air mixture velocity is less than the rate of flame propagation, then the flame has a tendency to retrogress down the port or aperture. In a premix-type burner, this retrogression could cause ignition within the burner body or the manifold. The burner assembly of the present invention provides a method of controlling the gas/air mixture velocity gradient, which controls lift-off of the flame and eliminates flashback through a wide range of percentages of stoichiometric combustion, gas/air mixture temperatures and turndown. Further, in most embodiments of this invention, the gas and air are not premixed in a combustible ratio at a mixture temperature above approximately 800.degree. F.
In the case of line burners employing present technology, the input (BTUH) per linear foot might be decreased to the approximate energy requirement per foot of oven length, but there are other limitations in the use of conventional line burners when the total length exceeds 7 to 10 feet. As the mixture velocity increases through the body of the burner housing, the distribution of the gas/air mixture is affected, resulting in non-uniform burning. A further limitation of these line burners is that when long burner sections are interconnected, the burners have a tendency to arc or bow due to thermal expansion. These burners are further limited in that they are sensitive to the gas/air ratio.
The limitations of the present day line burners, and also of packaged-type burners, necessarily limits the performance of industrial ovens which utilize such burners. For example, I have developed the High Heat Transfer Oven of U.S. Pat. No. 4,235,023, the Radiant Wall Oven and Process of Drying Coated Objects of U.S. Pat. No. 4,546,553, and the Convection Stabilizied Radiant Oven (AIRRADIANT.TM. Oven) of U.S. Pat. No. 4,785,552. In the radiant-type ovens, conventional packaged-type burners have been employed, which release the energy of combustion in a rather confined space. Methods utilizing fans for distributing this energy are employed, which in one form or another distributes uniform heated air to the backside of the emitter walls. In the High Heat Transfer Oven, the mass movement of the air from the fans distributes the heat from the individual packaged burners well, but multiple burners and manifolds are required based upon the length of the High Heat Transfer Oven, which additional equipment increases the oven's cost.
While the designs described by these patents have proven to be highly efficient in maintaining uniform temperature on the surfaces of a vehicle or other objects passing through the respective oven, fans are required to distribute the heated air over the inner side of the emitter surfaces of the RADIANT WALL.TM. and AIRRADIANT.TM. ovens, and multiple burners are usually required in the High Heat Transfer Oven. A desirable and beneficial improvement in these ovens could result if the heat of combustion could be distributed over the inner emitter wall surface without the requirement of fans. Also, multiple burners could be eliminated in the High Heat Transfer Oven if the heat of combustion could be uniformly discharged throughout the oven length. The development of the burner assembly of the present invention provides a method by which heat can be transferred to the emitter walls in ovens described by U.S. Pat. No. 4,546,553 and U.S. Pat. No. 4,785,552, without the requirement of circulating fans in heater houses or within the internal cavity of the RADIANT WALL.TM. module. Also, the burner assembly of the present invention can uniformly distribute the heat of combustion throughout the full length of a High Heat Transfer Oven.
This burner assembly will have many other applications where it is desirable to release the energy of combustion over a long span, or where the temperature of the burner environment is highly elevated. While a present-day line burner has limitations as to the operational length, the burner of the present invention is capable of firing essentially any length of emitter wall or High Heat Transfer Oven. The limiting factor of length would not be because of distribution of the gas or air, or based upon thermal expansion and contraction problems, but based upon the time required for the flame to carry from the point of ignition to the other end of the burner. The burner assembly (burner) of the present invention overcomes these limitations and other problems that now exist with conventional line burners, and provides additional operational benefits, disclosed herein.